I'll be converting to an 8 rib shortly. I bought all my pullies from a mechanic who had them on his Cobra. I think most are off of a F-150. Check with a dealership for prices, then check with aftermarket companies like RR or ASP. If you're gonna really climb in the RPM range, consider a SFI certified crank pulley.
Also, with the 8 rib, you may have to use washers to act as a spacer to move your S/C out to be aligned with the wider pullies.